Monolithic inductors have been a challenging puzzle for engineers and academia alike. In order to manufacture a high quality inductor with minimal resistive loss, a sufficiently wide metal trace in the form of a spiral is laid down on the silicon. However, the enlarged area of the metal trace increases the parasitic capacitance between the metal trace and the substrate. The result is reduced self-resonance frequency of the inductor, which greatly limits the operating range of the inductor.
One proposed solution to this problem is to use special processing steps to form a very thick dielectric layer beneath the metal trace of the inductor. This is a non-conventional processing step that would add substantial fabrication complexity and increase the cost thereof.
Another proposal to improve the self-resonance frequency of the monolithic inductor is to create an air gap between the metal trace and the silicon substrate by etching away the silicon beneath the inductor. The inductor is effectively suspended without support underneath. This is a dubious solution because questions remain concerning the fragile structure, packaging, and general reliability of the device.